The Marauder's Map, a very memorable item from the Harry Potter movies.

Not only does it show every classroom, every hallway, and every corner of the castle, but it also shows every inch of the grounds, as well as all the secret passages that are hidden within its walls and the location of every person in the grounds, portrayed by a dot. It is also capable of accurately identifying each person, and is not fooled by animagi, Polyjuice Potions, or invisibility cloaks; even the Hogwarts ghosts are not exempt from this.

In the story, the map is created by Remus Lupin, Peter Pettigrew, Sirius Black and James Porter.